Why was it so important to keep the fire burning?
Ralph is sure that his father, who is in the Navy, will come to rescue them. Piggy isn't so sure. There has been an atomic attack on the world, and Piggy doesn't think anyone is alive to rescue them. However, Ralph says,
"We can help them to find us. If a ship comes near the island they may not notice us. So we must make smoke on top of the mountain. We must make a fire." (pg 38)
The purpose of the fire is to attract ships. When a ship does appear on the horizon, Ralph is sure that they will see the smoke and be rescued. But the children go hunting with Jack and let the fire die. This action causes a leadership crisis. Ralph emphasizes,
"The fire is the most important thing on the island. How can we ever be rescued except by luck, if we don't keep a fire going? Is a fire too much for us to make?" (pg 80)
The ironic aspect of this is that when all chaos breaks out on the island, and the children are hunting Ralph, they build a fire to try and smoke Ralph out, and the smoke does attract a ship. The officer who meets Ralph at the beach says,
"We saw your smoke. What have you been doing? Having a war or something?" (pg 201)